This invention relates to cleaning of contaminated liquid by centrifugal separation of denser contaminants, particularly separating out solids found in liquids such as lubricants used in internal combustion engines.
The use of centrifugal separators for such lubricant cleaning is well known in the art and has become particularly important as more vehicles use as fuel diesel oil that produces fine soot like particles and have to operate for considerable intervals without servicing of lubricant cleaning apparatus and change of lubricant.
Separating out solid contaminants depends upon passing the liquid though a rotating separation and containment vessel and the efficiency of separation is related both to the speed of rotation and time spent by the liquid in transiting the vessel.
Such contaminants make it necessary to rotate the centrifuge separation and containment vessel at higher speed than previously needed in the art, although this in turn has led to more complex designs. Whereas traditional constructions relied upon the liquid passing through the rotor container filling it and at such pressure as to create a reaction thrust as it was ejected by way of reaction jet nozzles, this was perceived to be unable to rotate a container filled with liquid adequately, the through-flow required for high speed rotation and radial pressure gradient across the container being in conflict with obtaining proper dwell time of the liquid within the container to effect separation. In order to achieve better rotation it has been suggested to separate the liquid driving the rotor from that passing through the rotor for cleaning. GB 2297499 employs liquid from a high pressure source separate from that being cleaned in the rotor. U.S. Pat. No. 6,454,694 describes splitting the contaminated liquid supplied to the rotor at elevated pressure and diverting a portion of it directly to reaction jet nozzles to supplement that passed through the separation and containment chamber. Other devices, such as EP 0980714 A2 employ an impulse turbine rather than liquid jet reaction turbine to effect high speed rotation.
Although such separate driving enables a more relaxed dwell time to be achieved for the liquid to be cleaned, it will be seen from the above that such arrangements can result in complicated, and thus expensive, rotor constructions. Furthermore, it is believed by inter alia the present applicant that such efforts to increase efficiency by increasing the rotation speed are nevertheless compromised by having to rotate a vessel filled with liquid. Such a filled vessel not only has considerable inertia that is in conflict with frequent starting and stopping of the engine, but retains the internal pressure gradient profile that conflicts with efficient separation unless internal structural features are employed that reduce throughput of the liquid.
Such rotor types which are operated filled with liquid and exhibit a pressure difference between the contained liquid and the surrounding environment may be considered as being of a “closed” rotor type,
Traditionally the cost of providing such centrifugal cleaning has confined its application to commercial vehicles, such as trucks, whose high degree of usage between servicing intervals is combined with the use of diesel engines that introduce particulate products of combustion into the lubricating oil. However, as such engines become more commonplace in passenger vehicles the benefits of centrifugal separation in engines of these vehicles are manifest as the lesser usage is more than offset by the requirement for long intervals between servicing, but such smaller and competitively priced vehicles place their own constraints on the overall cost of employing such separation in addition to conventional filtration.
The costs associated with having such a separator in the lubricant cleaning system of a vehicle include not only the directly quantifiable ones of manufacture and disposal in relation to the cost of the vehicle per se but also the less quantifiable ones of added weight to be carried around by the vehicle and power consumed in operation. It has been proposed for, example in DE 19715661 A1, to have such a closed type of rotor manufactured from synthetic resin (i.e., plastic) materials instead of the usual steel sheet, but the stresses imposed by rotating a large body of contained liquid require reinforcement that detracts from the apparent simplicity of substitution by a lighter material.
It has been proposed, as in WO 02/055207, the contents of which are incorporated herein by reference, to employ a so-called “open” rotor in which the liquid to be cleaned does not fill the whole chamber created by the rotor contaminant separation and containment vessel but is confined to a radially relatively thin zone or layer adjacent the vessel outer wall by virtue of outlet passage separated from the rotation axis and through which liquid is capable of being discharged at a rate in excess of its supply to the vessel. Liquid freshly introduced into the vessel is able to reach the zone adjacent outer wall, where separation is most efficient, without the pressure gradient of a filled rotor and the inertia of the rotor is reduced in accordance with the reduced amount of liquid held.
Having determined a more satisfactory principle of operation there nevertheless remains the problem of providing such cleaning benefits economically when there are cost constraints associated with particular vehicular types. As mentioned above, the costs associated with having a centrifugal separator in the lubricant cleaning system of a small passenger vehicle thus include both the costs of manufacture and disposal and of operation.
Whereas some of the indirect costs of operating may be mitigated by the “open” rotor approach and wide choice of materials suitable, there is still the need to consider construction in respect of initial manufacture and in respect of disposal that is now governed by legislation relating to separation and recycling of different materials.